Imaging sensors are light sensitive devices that convert light into electrical charges. Imaging sensors used in most digital imaging systems are charge-coupled devices (CCD) or complementary metal oxide semiconductor (CMOS) devices. These devices are typically composed of an array of light sensitive diodes called pixels that convert photons into electrons when exposed to light. The electrical charge that accumulates at each pixel on the imaging sensor array is proportional to the brightness of the light that reaches the pixel during the period of exposure. An electrical signal output from the device represents the collection of charges from the pixels and is used to generate an image.
Commercially available CCD and CMOS sensors have a limited range of light levels they can linearly convert into image pixels. This linear range can be adjusted by altering combinations of image acquisition property settings of image acquisition hardware, such as a video camera. Acquisition property settings comprise those properties that alter the acquisition hardware and, thus, the linear range of the image acquisition system. The properties include, but are not limited to, exposure time, gain, offset, contrast, and brightness. Acquisition property settings also comprise properties controlling the light levels in the scene. Such properties include, but are not limited to, controlling the illumination provided by fixed lights, strobe lights, or structured lighting. Light levels outside the linear range are clamped or clipped to the minimum or maximum electrical signal.
For example, FIG. 1 is a diagram illustrating the manner in which the range of light levels in a scene 10 typically corresponds to a pixel intensity value 20. Although most image sensors represent pixel intensity as an 8-bit value, the range of pixel intensity values 30 is shown as a 3-bit value for clarity. The light levels in the scene 10 that fall within the linear range are differentiable in an acquired image. In other words, within the linear range, pixel intensity varies proportionally to the light level in the scene. The range of light levels in the scene that fall outside the linear range will be either 0 or 7 and, therefore, not differentiable in an image. Setting the linear range too small relative to the light levels in the scene being acquired causes many of the objects in the scene to appear black or white with no grayscale information. Setting the linear range too large relative to the light levels in the scene causes many of the objects to also lose grayscale information as a single pixel intensity value represents a large range of light levels.
It should be noted that the conversion of photons to an electrical signal is inherently linear in silicon based sensors. The conversion of photons to an electrical signal in other sensor technologies may not be linear. The remainder of this application assumes the conversion is linear to make the description of the invention clearer.
Auto-exposure is a process for dynamically setting the acquisition property settings according to the pixel intensity values of one or more recently acquired images. In other words, auto-exposure is the automated process of choosing the correct combination of exposure setting, gain setting, offset and other property settings to yield an optimal linear range for the machine vision application. The purpose of auto-exposure is to improve the quality of acquired images and, in turn, to improve one or more aspects of the performance of the machine vision application using the acquired images. Auto-exposure often improves the accuracy, robustness, reliability, speed, and/or usability of a machine vision application. Traditional auto-exposure techniques include the various methods for performing auto-exposure known to those skilled in the art.